The present invention pertains generally to devices and methods for separating minority isotopes from majority isotopes. More particularly, the present invention pertains to functional parameters and dimensional considerations for a long plasma centrifuge that allows the device to be operated as an isotope separator. The present invention is particularly, but not exclusively, useful as devices and methods for separating isotopes when the difference between the mass numbers of the isotopes is relatively minimal (xcex94M/M less than  less than 1).
Whenever charged particles (e.g. plasmas containing electrons and positively charged ions) are subjected to crossed electric and magnetic fields (Exc3x97B), a rotational energy is imparted to the particles that causes them to rotate around an axis. This axis is defined by the relative orientation of the electric and magnetic fields, and the rotational energy that is imparted to the particles is determined by the magnitudes of the respective electrical and magnetic fields. During the rotation of charged particles in crossed electric and magnetic fields, collisions between the particles will xe2x80x9cheatxe2x80x9d the particles to an energy comparable to the rotation energy. The electron temperature (Te) in the plasma can be controlled independently and is chosen to be 1-5 eV giving a ratio of ion temperature to electron temperature of about one hundred to one (Ti/Te=100/1). Stated differently, with a given rotational energy for the ions in a rotating plasma, the ion temperature will be around one hundred times hotter than the electrons in the plasma.
It is also well known that plasma centrifuges, which rely on the rotational phenomenon mentioned above, can be used to separate charged particles from each other. Recently, it has also become known that plasma mass filters, such as disclosed in U.S. Pat. No. 6,096,220 which issued to Ohkawa for an invention entitled xe2x80x9cPlasma Mass Filterxe2x80x9d (hereinafter the xe2x80x9cOhkawa Patentxe2x80x9d), and which is assigned to the same assignee as the present invention, can be used for the same purposes. With either type device, centrifuge or filter, the difference between the masses of the particles that are to be separated is a significant factor for consideration. This consideration is important in both the design and operation of the device, and becomes more significant as the difference in mass between particles becomes less.
By definition, the mass number of an atom, xe2x80x9cMxe2x80x9d, is the total number of protons and neutrons in its nucleus. Also, by definition, an isotope is one of a set of chemically identical species of an atom which have the same atomic number (i.e. same number of protons), but which have different mass numbers (i.e. a different number of neutrons). Further, it can happen that a material will include different isotopes. When this happens, it may be desirable for a variety of reasons to manipulate the material by separating its minority isotope ions from its majority isotope ions.
Mathematically, it can be shown that the separation of minority isotope ions from majority isotope ions in a plasma can be quantified by a separation factor xe2x80x9cxcex5xe2x80x9d which is expressed as:
xcex5=exp{xcex94Mxcfx892r2/2kBT}
where, xcex94M is the mass difference between the minority and majority isotopes, xe2x80x9cxcfx89xe2x80x9d is the angular rotational frequency of the plasma, xe2x80x9crxe2x80x9d is the radial distance of a particle from the axis of rotation, kB is Boltzmann""s constant, and xe2x80x9cTxe2x80x9d is the ion temperature in the plasma.
From the above expression for the separation factor xe2x80x9cxcex5xe2x80x9d, several particulars affecting the separation of charged particles in a rotating plasma can be appreciated. First, it is to be noted that the separation factor xe2x80x9cxcex5xe2x80x9d is directly proportional to xcex94M. Accordingly, for the separation of isotopes, where xcex94M is typically small, separation will be inherently more difficult than when different elements are involved. Despite this observation, however, it would initially appear that greater separation efficiency can be achieved merely by increasing the rotational energy of the plasma (rotational energy is proportional to xe2x80x9cxcfx892xe2x80x9d). It turns out the situation is not quite so simple.
Increasing the rotational energy to increase the separation efficiency of a gaseous or plasma centrifuge has its limitations. It happens that as the mass of particles to be separated is increased, more rotational energy is required. Particularly for high mass isotopes, where xcex94M/M less than  less than 1, significant rotational energy may be required to achieve effective separation. The maximum permissible rotational energy in a conventional gas centrifuge, however, is restricted by the strength of the rotor, and as a consequence, many passes through a centrifuge have been required to effectively separate minority isotope ions from majority isotope ions.
In addition to the above observations, it is also known that in plasma centrifuges, as the rotational energy of a plasma is increased, so too is its thermal energy. Consequently, according to the expression for the separation factor xe2x80x9cxcex5xe2x80x9d given above, when both rotational and thermal energy are increased together there may be little, if any, net gain in efficiency. In addition, the standard plasma centrifuge has two solutions: a high rotation frequency that is difficult to access, and the normal low rotation frequency solution. It can be shown that even with high rotation solution in the standard plasma centrifuge, the electrons are not rotating at the same velocity and the resultant azimuthal drag on the ions result in a radial drift that interferes with the mass separation. On the other hand, the above expression also indicates that an increase in efficiency can be achieved by lowering the temperature xe2x80x9cTxe2x80x9d of ions in the plasma.
In light of the above, it is an object of the present invention to provide an isotope separator that is effective for separating relatively high mass isotopes from lower mass isotopes when the difference between the mass numbers of the isotopes is relatively minimal (xcex94M/M less than  less than 1). Another object of the present invention is to provide an isotope separator that can effectively separate minority isotope ions from majority isotope ions in a single pass of the ions through the separator. Still another object of the present invention is to identify dimensional requirements, along with operational parameters, that will increase the efficiency of an isotope separator. Yet another object of the present invention is to provide an isotope separator that is relatively easy to manufacture, is simple to operate, and comparatively cost effective.
In accordance with the present invention, a device for separating isotope ions includes a cylindrical chamber which defines a longitudinal axis. Dimensionally, the chamber has a length xe2x80x9cLxe2x80x9d that extends along the axis between the ends of the chamber, and it has a radius xe2x80x9caxe2x80x9d that is measured from the axis. An injector for introducing a plasma into the chamber is mounted at an end of the chamber so that the plasma will travel the length xe2x80x9cLxe2x80x9d as the plasma transits through the chamber. For the present invention it is envisioned that the plasma will include both minority isotope ions having a mass number xe2x80x9cM1xe2x80x9d and majority isotope ions having a mass number xe2x80x9cM2xe2x80x9d. Further, it is envisioned that these mass numbers can be relatively high, with M1=M2+xcex94M, and M1≅M2≅M such that xcex94M/M  less than  less than 1. For the present invention, because M1≅M2, the mass number xe2x80x9cMxe2x80x9d of the majority isotope alone can be used for purposes of calculating operational parameters for the chamber.
For the operation of the present invention, crossed electric and magnetic fields (Exc3x97B) are established inside the chamber. Specifically, the crossed electric and magnetic fields are established with specific values to excite the plasma with a rotational energy. Preferably, the temperature of the isotope ions (Ti) inside the chamber, due to this rotational energy, will be in a range between approximately one hundred and four hundred electron volts (Ti=100 xcx9c400 eV). In contrast, the temperature of electrons in the plasma (Te), will be maintained in a range of approximately one to five electron volts (Te=1xcx9c5 eV).
For the specific case wherein a plasma centrifuge is being used to separate isotope ions, the crossed electric and magnetic fields are established so as to confine all of the isotopes on trajectories inside the plasma chamber as they transit the chamber. Thus, in accordance with the Ohkawa Patent, a so-called cut-off mass xe2x80x9cMcxe2x80x9d is established to be greater than xe2x80x9cMxe2x80x9d (the mass of the isotopes) for singly ionized ions. Preferably, for purposes of the present invention, xe2x80x9cMcxe2x80x9d will be selected to be approximately twice as high as xe2x80x9cMxe2x80x9d. Calculations can therefore be made using a value for the ratio M/Mc without necessarily calculating xe2x80x9cMcxe2x80x9d (e.g. M/Mc=0.5).
An important aspect of the present invention is that the electrons in the plasma are used to cool the isotope ions (recall; Te≅Ti/100). Specifically, this is done in order to effectively improve the separation factor xe2x80x9cxcex5xe2x80x9d set forth above.
As envisioned for the present invention, the cooling process and consequent improvement in the separation factor xe2x80x9cxcex5xe2x80x9d is accomplished by increasing the residence time, xcfx841, during which the plasma is held in the chamber. For the present invention, this is done by properly engineering the length xe2x80x9cLxe2x80x9d of the chamber. In particular, the length xe2x80x9cLxe2x80x9d is selected so that the residence time, xcfx841, is longer than the required cooling time, xcfx842.
In addition to merely lengthening the chamber in order to accomplish the purposes mentioned above, the present invention also envisions alternate embodiments for the chamber. For one alternate embodiment, the magnetic field xe2x80x9cBxe2x80x9d is manipulated. Specifically, the magnetic field is configured to have a strength xe2x80x9cBexe2x80x9d at both ends of the chamber, and a strength xe2x80x9cBmxe2x80x9d therebetween. In this embodiment, Be greater than Bm to increase the residence time, xcfx841.